Welding metallurgy part i

WELDING METALLURGY

ME 473 WELDING TECHNOLOGY

Instructor: Assist.Prof.Dr. Oğuzhan Yılmaz

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Basic Metallurgy The science of joining metals by welding that relates closely to the field of

metallurgy.

Metallurgy involves the science of producing metals from ores, of making and compounding alloys, and the reaction of metals to many different activities and situation.

Heat treatment (heating and cooling of metals to obtain desired shapes and mechanical properties)

Steel making and processing

Forging

Foundry

Welding metallurgy can be considered a special branch, since reaction

times are in the order of minutes, seconds, fraction of seconds, whereas in

the other branches reactions are in hours and minutes.

Welding metallurgy deals with the interaction of different metals and

interaction of metals with gases and chemicals of all types.

Dr. Oğuzhan Yılmaz

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Welding metallurgist will examine the changes in physical characteristics

that happen in short periods. The solubility of gases in metals and

between metals and the effect of impurities are all of major importance to

the welding metallurgist.

Basic Metallurgy


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The structure of metal is complex. When metal is in a liquid state, usually hot, it has no distinct structure or orderly arrangement of atoms. So that atoms move freely since they have high degrees of mobility due to the heat energy involved during melting process.

As the metal cools, atoms loose their energy and their mobility. When temperature is further reduced, the atoms are no longer able to move and attracted together into definite patterns.

These patterns consist of three-dimensional lattices, which are made of imaginary lines connecting atoms in symmetrical arrangements.

Basic Metallurgy_Crystalline structures

Metals in a solid state possess this uniform

arrangements, which is called crystals. All metals are

crystalline solids made of atoms arranged in a

specific uniform manner.


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There are three common types of lattices;

(1) The face-centered cubic

(2) The body-centered cubic

(3) The hexagonal close-packed

Iron has both FCC and BCC structures but at different temp. This is know as ‘allotropic change’.

The crystal lattices are only for pure metals that are composed of one type of atom. However, most metals that are common use are alloys (more than one metal).

In alloys, the crystals will change.

According to the portion of the alloy,

there are three types of formation

occur:

(1) substitutional solid solution.

(2) interstitial solid solution and

(3) intermetallic compounds.


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Substitutional solid solution: the atoms of the metal making up the minor portion of the alloy will at random replace some of the atoms of the metal making up the majority of the alloy.

Interstitial solid solution: The atoms of the minor metal in the alloy are much smaller than those in the major lattice, they do not replace the atoms of the major metal in the lattice but rather locate in points between or intervening spaces known as interstices in the lattice.

Intermetallic compounds: the minor metal atoms in the alloy cannot completely dissolve either interstitially or substitutionally. They will form the type of chemical compound the composition of which corresponds roughly to the chemical formula. This results in the formation of mixed kinds of atomic groupings consisting of different and complicated crystalline structure. [Fe3C, Cementite,Iron-Carbide]

Each group with its own crystalline structure is referred to as a phase.



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Different alloys, solid solutions, intermetallic compounds, and phases occur as the molten metal solidifies.

Solidification occurs in all direction which are normal to the nuclei crystal that is a small crystal form. For a cubic crystal, growth progress is in six direction simultaneously. Growth is simply the adding on of additional

crystals as tempereture decreases.


GRAIN

When the resultant structure is cut in

a flat plane, the individual dentritic

crystals, which grew until they met

adjacent dentritic crystals, form an

irregularly shaped area, known as a

‘grain’. Grains have boundaries and

are very small but much larger than

the individual crystals


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The size of the crystals and grains depends on the rate of growth of the

crystal. The rate of crystal growth depends on the rate of cooling of the

molten solidifying metal.

When the rate of cooling is high, the solidification process occurs more

rapidly and the crystal size and graing size tend to be smaller and vice

versa. (‘snow’ example)

Metal structures can be characterized as having large grains (coarse

grained) or small grains (fine grained) or a mixture of large and small

grains (mixed grain).

The arrangement of atoms is irregular in the grain boundaries, and there

are vacancies or missing atoms. The atom spacing may be larger than

normal, and individual atoms can move easily in the grain boundaries;

because of this, the diffusion of elements, which is the movement of

individual atoms through the solid structure, occurs more rapidly at grain

boundaries.

Basic Metallurgy_Grains


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Microstructure

The overall arrangement of grains, grain boundaries, phases present in an

alloy is called its microstructure. It is largely responsible for the properties

of the metal.

The microstructure is affected by the composition or alloy content and by

other factors such as hot or cold working, straining, heat treating etc.

The microstructure of weld metal and adjacent metal is greatly

influenced by the welding process, which influence the properties of

the weld.

Basic Metallurgy_Microstructures

Microstructure of a weld used in stainless steel Microstructure of base metal of the same stainless steel


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Some metals change their crystallographic arrangement with changes in

temp. Iron has a BCC lattice structure from room temp. up to 910ºC, and

from this point to 1388 ºC it is FCC. Above this point to melting point, 1538

ºC it is again BCC. This change is called as phase transformation or

allotropic transformation. Like, titanium, zirconium and cobalt.

Transformation occurs when metal melts or solidifies;

In melting, arrangement of atoms disappears and atoms

move randomly.

In solidifiying, crystalline arrangement reestablish itself.

Pure metals melts or solidify at a single temperature, while alloys solidify

or melt over a range of temperature with a few exceptions.

Phase changes can be related to alloy compositions and temp when they

are in equilibrium, and shown on a diagram (known as phase diagrams,

alloy equilibrium diagrams or constitution diagrams).

Basic Metallurgy_Phase transformation


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Equilibrium diagrams are used to determine the phases that are present

and the percentage of each, based on the alloy composition at a temp.

And changes by increasing and decreasing temp. Most of them are

designed for alloy system containing two elements.

In welding because of rapid changes in temperatures, equilibrium

conditions are rarely occur. In an equilibrium condition, the metal is stable

at the particular point on the diagram based on relatively slow heating and

cooling.

Basic Metallurgy_Phase transformation


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Iron-carbon equilibrium diagram provides an insight of the behaviour of

steels in connection with welding thermal cycles and heat treatment. This

diagram represents the alloy of iron with carbon, ranging from 0% to 5%

carbon.

Basic Metallurgy_Iron-Carbon diagram

0.25


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Pure iron is relatively weak but ductile metal. When carbon is added in

small amounts, the iron acquires a wide range of properties and uses and

becomes the most popular metal, ‘steel’.

0% carbon, pure iron,

above 1540ºC, in liquid state, no crystalline structure

< 1540 ºC, solidification starts, BCC structure, Delta iron

< 1400 ºC, transformation occurs, FCC structure, Gamma iron

< 910 ºC, iron back to BCC, alpha iron until room temp

Iron and carbon form a compound known as iron carbide (Fe3C) or

cementite.

When iron carbide or cementite is heated above 1115 ºC, it decomposes

into liquid iron saturated with graphite, which is a crystalline form of

carbon.



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Ferrite This phase has a Body Centre Cubic structure (B.C.C) which can

hold very little carbon; typically 0.0001% at room temperature. It can exist as

either: alpha or delta ferrite.

Austenite This phase is only possible in carbon steel at high

temperature. It has a Face Centre Cubic (F.C.C) atomic structure which can

contain up to 2% carbon in solution.

Cementite Unlike ferrite and austenite, cementite is a very hard intermetallic

compound consisting of 6.7% carbon and the remainder iron, its chemical

symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite

layers its average hardness is reduced considerably.

Pearlite A mixture of alternate strips of ferrite and cementite in a single

grain. The name for this structure is derived from its mother of pearl

appearance under a microscope. A fully pearlitic structure occurs at 0.8%

Carbon. It is a lamellar structure, which is relatively strong and ductile.



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Ferrite Pearlite

Austenite


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Consider a steel with a composition of 0.25% carbon. A vertical line is

drawn up at this point;

Above 1520ºC, the steel is molten, as the temp decreases, delta iron start to

form in the liquid.

Just below 1500 ºC, transformation to austenite and molten metal.

At about 1480 ºC, all the liquid metal solidifies and the form is austenite.

Approx. 815 ºC, the austenite begins to breakdown and form a new phase,

ferrite.

Ferrite formation continues until a temp 727 ºC

At 727 ºC, the remaining austenite structure would disappear completely and

transforming to a structure known as pearlite+ferrite

In welding the rise and fall of temp or the rate of change of temp is so fast

that equilibrium does not occur. Therefore, aforementioned structures will

be different.



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At fast cooling rates, the austenite might not have sufficient time to

transform completely to ferrite and pearlite and will provide a different

microstructure. In this case, some of the untransformed austenite will be

retained and the carbon is held at supersaturated state. This new structure

is called ‘martensite’.

If the cooling rate is sufficiently fast, the austenite might transform

completely martensite. It is harder than pearlite or ferrite-pearlite structure

and it has lower ductility.

Basic Metallurgy_Martensite formation


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Hardness mainly depends on the carbon content but cooling rate also influences the microstructure and causes higher hardness. This is because the crystal lattice is changed or distorted and this hardens the material.

By adding different alloys to the steel, the tendency of austenite to transform into martensite upon cooling increases, which is the basis of hardening steels. Carbon, manganese, chromium, molybdenum etc.

The amount of alloys and their power to create this microstructure transformation are known as hardenbility.

Grain size and microstructure relate directly to hardness and strength. Fine grain size promotes both increased in strength and hardness.

This is an advantage for heat treatment but it can be detrimental to welding since high hardness is not desired in welds of softer materials.

Basic Metallurgy_Hardenability


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The heat treatment of steels to increase hardness and the metallurgy of

welding have much in common.

Most steels possess the property of hardenability, which is defined as the

property that determines the depth and distribution of hardness induced by

quenching, and this property can be measured by the ‘quench-test’, that is

used to plot hardness value from quenched end to unquenched end.

Basic Metallurgy_Hardenability

The quench-test and the

information obtained provides

usefull data for welding since it

indicates the effect of different

alloying elements on the

hardness of the quenched

steel. The microstructure of the

quenched steel can also be

studied and related to the

microstructure of welds.


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When a weld is made, following factors occur:

The changes of temperature

The growth of dimensions

The phase transformation etc.

The rate of cooling or quench is of primary importance and this is

controlled by the process, procedure, metal and mass.

Welding Metallurgy

Example: The electroslag has the

lowest cooling rate among welding

methods, while the gas metal arc

has a much faster cooling rate.


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The rate of change decreases as the distance from the center of the weld increases.

Welding Metallurgy

It is obvious that many different

cooling rates occur and that

different microstructures will result.

Also different phases occur in the

base metal adjacent to the weld.

(a) Mixture of ferrite and pearlite

grains

(b) Pearlite transformed to Austenite

(c) Full Austenite transformation

(d) Completely liquid state


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In addition to the complications created by the rapid cooling, there is also

the complication of composition variations.

As weld metal is deposited on a base metal, some of the base metal melts

and mixes with the weld metal, producing a dilution of metal.

If the compositions of the weld metal and the base metal are not identical,

variation of composition at the interface can be observed and also it

causes variation of cooling rates. This results variation of microstructures.

Welding Metallurgy


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Welding Metallurgy


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Each microstructure has its particular characteristics and one of the

important characteristics is the hardness of the microstructure throughout

the weld area.

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The area between the interface of the deposited weld metal, and

extending into the base metal far enough that any phase change occurs, is

know as the heat-affected-zone (HAZ).

HAZ is a portion of the weld since it influences the sevice life of the weld.

HAZ is the most critical in many welds. For instance, when welding a

hardenable steel, HAZ can increase in hardness to an undesirable level.

When welding a hardened steel, HAZ can become a softened zone since

the heat of the weld has annealed the hardended metal.

Welding Metallurgy_Heat affected zone

Heat-affected-zone (HAZ)

weld


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It may occur in two possible ways, (1) migration of oxides along the grain

boundaries rendering them weak. (2) oxidation as in oxygen cutting.

Protections are carefully supplied to exclude the atmosphere from the

high-temperature welding regions. Protective agents are usually in the

form of inert gases, fluxes, and electrode coatings.

Metallurgical problems in welding_Burning


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Segregation is one of the important factor that should be considered. It

relates the solubility of elements in metals, particularly alloys.

For instance, the composition of the first crystals that form as an alloy

freezes is different from the composition of the liquid that freezes last.

In weld metal, because of the rapidity of freezing time, very little diffusion

occurs and there is a lack of homogeneity in the total weld.

Carbon, phosphorus, sulfur and sometimes manganese are frequently in the

segregated state in steel. This can be determined by high-magnification

study of the microstructure.

Metallurgical problems in welding_Segregation

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Molten metal has a relatively high capacity of dissolving gases in contact with it. As the metal cools it has less capacity for dissolved gases, and when going from liquid to solid state the solubility of gas in metal is much lower.

The gas is rejected as the crystals solidify, but it may be trapped because of almost instantaneous solidification. Entrapment of the gas causes gas pockets and porosity in the weld.

Carbon monoxide, which is present in many arc and fuel gas atmospheres, is sometimes trapped. Hydrogen can also be trapped but it may gradually disperse and escape from the weld metal over a period of time. High temp increases the speed for hydrogen migration and removal.

The inert gases are not soluble in molten metal and for this reason, they are used in many gas shielded applications.

The solubility of metals within metals is also crucial. The greater the degree of solubility, the better the success of welding dissimilar metal combinations.

Metallurgical problems in welding_Gas pockets

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Welding metallurgy part i